This invention relates to a method of controlling several clocked loads that can be controlled individually and independently of each other, more particularly electrical heating loads of an electric oven, for minimizing the demand on an alternating current mains due to switching transients, more particularly due to flickering. The loads are fed from a common phase of the alternating current mains and are switched on each time at the beginning of their clock period. The clock periods of the various loads are of equal length.
When controlling the electrical loads of an electric oven, i.e. for example of the oven plates, it is generally deemed sufficient to switch the loads periodically on and off by means of a bi-metal relay. The clock periods are then generally long so that the power and temperature fluctuations resulting therefrom are not always compensated for satisfactorily by the thermal inertia of the system, more particularly of the oven plate itself. Due to the use of mechanical relays, the high-frequency interference caused by contact sparks and/or by switching outside the zero voltage passages increase with increasing frequency.
This problem is avoided by a known electronic circuit which switches the loads at the voltage or current zero passages by means of semiconductor elements and thus drastically reduces high-frequency interference. The loads are then switched on for whole multiples of the mains period, the transient time being typically several seconds. A substantial shortening of the clock periods is not possible in this circuit because an excessively high demand due to load variations and an excessively strong flicering then occur. The term "flickering" is to be understood to mean disturbing light fluctuations of lamps which are produced by the clocking of power loads at the same phase of the mains. Clock frequencies below 25 Hz are already cleared observed, but those in the range of about 10 Hz are found to be particularly disturbing. The international standard for the admissible flickering dose is defined in the IEC Publication 533 (Disturbances in supply systems caused by household appliances and similar electrical equipment). In this standard, each load variation is associated with a so-called recovery time whose duration depends upon the value of the load variation. The standard implies that during an observation time of from 5 to 15 minutes the sum of the recovery times (corresponding to the load variations occurring during the observation time) must not be larger than the observation time itself.
U.S. Pat. No. 4,256,951 discloses a power control circuit in which each load is switched on for a mains period. If, for example, four loads are switched by the control circuit, an individual heating load is switched on at full power during every forth mainx period, while it is switched on at half power during every eighth mains period, etc. In this switching mode of the loads, the flickering is not optimized. The flickering is further intensified in that the individual load should be designed for the multiple power with regard to its current consumption. This is obtained in that the load at full power is switched on only for a part of the time so that at a desired heating power of, for example, 2 kW and four loads to be switched the heater coil must have a current consumption corresponding to 8 L kW. This 8 kW load is switched on at full power during every fourth mains period. This is very unfavourable with regard to the flickering. Furthermore, the heater coils must have unnecessarily large dimensions.